1. Field of the Invention
The present invention relates to an image sensing device using a charge transfer type image sensing element, and provides an image sensing device which can discharge unwanted charge at a high speed.
2. Description of the Prior Art
Conventionally, a two-dimensional CCD (Charge Coupled Device) of frame transfer and interline type for obtaining image signals, and a one dimensional image sensing element used for a sensor device, have the drawback that, prior to starting to storing an image, the photoelectrons and thermoelectrons which have been stored before must be cleared, since otherwise those unwanted electrons result in noise lowering the S/N ratio.
In order to eliminate this drawback, it has been proposed that a switching element be provided for each of the picture elements of the image sensing element, and the switching elements all be actuated to discharge the charge stored in the picture elements to a power source. However, such as arrangement having the switching elements for respective picture elements image sensing elements leads to an increase in the number of the circuit elements, making the IC structure difficult. Further, the IC chip area and cost of manufacture increase, too. Moreover, the provision of such a switching element in the vicinity of the picture element necessarily reduces the area which can be occupied by the picture elements, thus decreasing the sensitivity of the image sensing device as a whole.
Therefore, in the conventional machines, the clearing operation is effected by reading out the charge information by a usual scanning process immediately before the start of the storing operation. However, this requires much time, and what is more, several clearing operations are needed to sufficiently clear a saturated CCD. Therefore, when the image sensing device is triggered, the scanning is first effected for a one frame picture to clear the charge, and then the storing for a new image is started. If the object to be sensed is moving at a relatively high speed, no desirable image can be provided because of the gap between the triggering and the start of image sensing. If the object has a high brightness, the storing time must be reduced correspondingly, since otherwise the charge in the image sensing element is saturated. In this case, if the storing time for the new image is shorter than the clearing time, the dark current and photoelectrons generated during the clearing time greatly affect the S/N ratio.